ERA and CARLA Detailed

ERA and CARLA use ROS to communicate each other. Standalone ERA currently does not have a way to read from or write to CARLA however it is not a complicated task to add that functionality.

How the system works?

CARLA is a driving simulator using unreal engine to simulate environments, vehicles, pedestrians, and sensors. It has a server-client architecture which means that you need to run Carla as a standalone application and interact with it through its API. CARLA's API is written in Python.

CARLA also has a ROS interface to easily interact with the ROS world. It is called ros-bridge. It basically uses CARLA's API and translate the sensor and position data to ROS topics. It also allows ROS nodes to control vehicles in CARLA using velocity commands.

ERA (master branch) uses ros-bridge (not to be confused with ros-bridge) to communicate with CARLA. CARLA runs, in a ROS independent fashion, separately in a single process. ros-bridge communicates with CARLA through sockets and exposes sensor data as ROS topics.

Current version of ERA only reads lidar data, camera data, and vehicle position data.

How to install ERA with CARLA?

Follow this wiki page.

As a summary, you need to install CARLA (apt-get), ros-bridge, and ERA ROS version.

How to run the system?

Running the system is straight forward:

• First run CARLA in a separate terminal: /opt/carla-simulator/bin/CarlaUE4.sh
• Run ERA in another terminal: roslaunch era_gazebo carla_two_cars.launch
• Finally you may want to run rviz in a third terminal for visualization.

What is happening under the hood?

roslaunch era_gazebo carla_two_cars.launch does many things and launches a bunch of ros nodes in parallel.

• It tells Carla to load Town10HD environment. This is a downtown environment with many two lane roads and intersections. You can find available maps here
• It loads two cars with specific models. For each car you need to specify its models. The list of models can be found here.
• For each car you need to specify a start location in the format of x,y,z,roll,pitch,yaw.
• Once the environment and vehicles are decided, the launch file runs $(find carla_ros_bridge)/launch/carla_ros_bridge.launch. This launch file runs ros-bridge. You only need one ros-bridge as it will take care of all the vehicles.
• For each car, the file runs through the following steps:
  • $(find carla_ego_vehicle)/launch/carla_example_ego_vehicle.launch: It spawns the vehicles at the given positions
  • $(find carla_manual_control)/launch/carla_manual_control.launch: It launch some controllers to help you move a car through ROS interface
  • $(find carla_ackermann_control)/launch/carla_ackermann_control.launch: It enables ackermann controls and commands.
  • costmap_2d_node: Our costmap node. Creates a costmap for a given car
  • $(find era_gazebo)/launch/v2v.launch: enables v2v communication. It runs the gnuradio communication layer between vehicles
  • $(find era_gazebo)/launch/object_detection.launch: It runs object detection using the front camera image.

Creating Scenarios

This is the tricky and somewhat painful part of the process. Unfortunately there is no easy way to create scenarios. A lot of trial and error is used in the process.

The scenario is defined in the python script era_gazebo/scripts/spawn_carla_npc.py. This file is not a ROS file and communicates directly with CARLA through its python API.